A typical coating finish over a substrate comprises some or all of the following layers: (1) one or more primer layers that provide adhesion and basic protection, and also cover minor surface unevenness of the substrate; (2) one or more basecoat layers, typically pigmented, that provide durability and color; and (3) one or more clearcoat layers that provide additional durability and improved appearance. A colored topcoat layer can be used in place of the basecoat layer and clearcoat layer.
In some industrial applications, such as coating metal pipes, trucks, and other industrial equipment, it is often desired to complete the coating process in a short period of time while still achieving good adhesion, protection, durability and appearance. Conventional coating compositions for colored layers are often less optimized for good adhesion directly to metals. In addition, a conventional coating composition typically produces a thin dried and cured coating layer that may not have sufficient thickness to cover unevenness of the substrate if only a single layer is used. Coating unevenness could result in undesired appearance. When conventional coatings are applied at a high coating thickness, surface coating defects such as microfoaming, low gloss, and sagging may occur. Thick coating layers can be prone to sagging defects, especially for coating layers applied on vertical surfaces.
Because of the known sagging problems when applying coatings, sag control agents are often added to form a smooth sag-free surface. Typical sag control agents are produced reacting a compound containing one or more amine groups with a compound containing one or more isocyanate groups, generally in the presence of a film forming binder, such as, for example, an acrylic resin. Because film forming resins are viscous in nature, the resins generally contain an organic solvent to lower the viscosity to a level that is suitable for the formation of the sag control agent. This adds additional volatile organic content (VOC) to the sag control composition and ultimately to the coating composition.
Therefore, there is a need for sag control agents and sag control compositions that can provide low VOC coating compositions that are sag-free at high coating thicknesses, having good direct-to-metal adhesion, and good appearance.